Railroad draft gear shock absorbing units have undergone an evolutionary state of development. Indeed, the shock absorbing material must be extremely resistant to softening under repetitive shock loads. A softened shock absorber doesn't absorb enough shock. A simple substitution of materials has been found to be ineffective.
Of particular importance is resistance to softening under demanding requirements of a dynamic compressive cycling test as a measure for determining suitable units for railroad draft gear service. This test requirement is in addition to typical deflection, low temperature and drop hammer tests involving visual destruction of a unit. Indeed, the softening test is more subtle, since a unit can soften without visual signs of disintegration. The softening determination aspect of a compressive cycling test can be exemplified by obtaining a resilient shock absorbing element comprising a resilient composition of a short cylindrical shape measuring about 16.51 centimeters in diameter and about 3.7 centimeters high with its sides in the shape of a concave V and with its ends covered and adhered to the face of circular steel plates and cycling said element under substantially constant compression alternating between a maximum of about 45 to about 55 percent of its original uncompressed polyurethane element height and a minimum of about 8 to about 12 percent of its said original height. Thus, the unit is always under some degree of compression with about 55 percent being the maximum. The force necessary to achieve the maximum compression (about 55 percent) is measured.
The polyurethane element itself constantly undergoes a very substantial change in shape as it is compressed and decompressed during each dynamic cycle. Under this relatively severe test, a typical unit can break down or crack within about 50 to about 100 cycles. However, a suitable unit for railroad draft gear service should last or withstand at least about 500 cycles.
A suitale unit for railroad draft gear service should maintain its compression resistance, or resistance to softening, by requiring at least about 37.times.10.sup.6 Newtons per square meter to compress the unit 55 percent of its original uncompressed height after about 500 cycles of the compressive cycling test.
Therefore, in view of these substantial and demanding shock absorbing compressive endurance requirements, it is an object of this invention to provide an improved resilient shock absorbing unit.